1. Field of the Invention
The present invention relates generally to probes for monitoring plasmas during semiconductor device fabrication processes and, more specifically, to probes that are used to monitor plasma characteristics during semiconductor device fabrication processes. More particularly, the present invention relates to probes that may be used to monitor plasma characteristics in such a manner as to generate a three-dimensional representation of the state of a semiconductor substrate being exposed to the plasma. In addition, the present invention relates to probes that are in substantially the same electrical state as a semiconductor substrate exposed to the same or a similar plasma. The present invention also relates to methods for fabricating the probes of the present invention, as well as to methods for evaluating one or more characteristics of a plasma and the corresponding effects thereof on a semiconductor substrate.
2. State of the Art
Conventionally, plasma processes have been used to deposit materials onto substrate surfaces, as well as to remove materials from substrate surfaces. With respect to the use of plasmas in semiconductor device fabrication processes, some chemical vapor deposition (CVD) processes, which are commonly referred to as plasma-enhanced chemical vapor deposition (PECVD) processes, ion implantation processes, and dry etch processes (e.g., reactive ion etching (RIE)) each employ plasmas. When plasma processes are employed to deposit material onto or remove material from a substrate surface, the plasma may generate electric potentials on the surface. The electric potential generated by the plasma is defined by the energy of the ions and electrons in the plasma and the rate that such ions and electrons arrive at the surface during processing.
The electric potential at the substrate of a semiconductor device is important to define the condition and consistency of the plasma processing being used and the quality of the subsequent substrate. Thus, monitoring of the plasma potential may be used to monitor and improve semiconductor device quality.
Various techniques for monitoring the effects of plasmas on substrates have been developed, as have mechanisms for reducing the potentially damaging effects of plasmas on the delicate features of semiconductor device structures.
Conventionally, so-called Langmuir probes have been used to monitor various properties of plasmas, including electron density (ne), electron temperature (Te), and plasma potential (Vp). Langmuir probes typically include a small electrode that communicates with a power supply. When the electrode is placed in a plasma, the power supply may be used to bias the electrode to various potentials with respect to the plasma. By measuring the current that flows through the electrode and power supply, information on properties of the plasma within the vicinity of the electrode may be measured.
While conventional Langmuir probes include single electrodes and, thus, may only be used in evaluating the properties of a plasma at a single location thereof, state-of-the-art Langmuir probes include probe arrays, the use of which facilitates evaluation of a plasma at several locations. These state-of-the-art probe arrays typically include a number of identical, miniaturized Langmuir probes that are held into position with respect to one another by a planar substrate. In one exemplary probe array, the probes are spaced about one centimeter from one another.
Due to the extremely small dimensions of semiconductor device features, neither conventional Langmuir probes nor the state-of-the-art probe arrays are equipped to provide an accurate analysis of a plasma at the locations where plasma processes are being conducted upon a semiconductor device structure.
Further, the characteristics of a plasma are determined, at least in part, by conditions within the plasma, including a material or materials upon which plasma processes are being conducted. As the materials from which conventional and state-of-the-art Langmuir probes are different from the materials of semiconductor device structures, a plasma's characteristics may be much different in the presence of a conventional or even a state-of-the-art Langmuir probe than they would be in the presence of a semiconductor device structure.
Plasma sensors have been developed with the intent of simulating a plasma-processed wafer when subjected to a plasma. This type of plasma sensor includes the so-called “CHARM®” sensor disclosed in Lukasek, et al., “CHARM®, a New Wafer Surface Charge Monitor,” Tech Con '90, San Jose (hereinafter “Lukasek 1”), and the “CHARM-2” sensor disclosed in U.S. Pat. No. 5,315,145, issued to Lukasek on May 24, 1994 (hereinafter “Lukasek 2”). These sensors store data representative of the charge generated by a plasma at various locations thereof, which data may be evaluated only after the plasma processes have been conducted.
CHARM® plasma sensors include electrically erasable programmable read-only memory (EEPROM) transistors that collect and store data representative of a charge generated by a region of a plasma to which these transistors are subjected. Nonetheless, as indicated by Lukasek 2, the EEPROM transistors of CHARM® plasma sensors store charge cumulatively (i.e., added together). By way of example, if an EEPROM transistor at a particular location of a CHARM® plasma sensor is subjected to a region of a plasma that generates a negative potential and subsequently subjected to a region of a plasma that generates a positive potential, the amount of charge stored by that EEPROM transistor will be the sum of the negative and positive potentials. Thus, the EEPROM transistors of a CHARM® plasma sensor may not accurately represent the largest positive or negative potentials that were generated by regions of a plasma to which such transistors were subjected. Consequently, CHARM® plasma sensors may not accurately indicate plasma conditions which may result in damage to semiconductor device structures during fabrication thereof. Further, CHARM® plasma sensors are only capable of monitoring plasmas in two dimensions.
CHARM-2 plasma sensors are useful for monitoring both the negative and positive transient effects of a plasma. Diodes or combinations of diodes and resistors are provided in series between the electrodes at which plasma characteristics (e.g., voltage generation) are monitored and the EEPROM transistor of a CHARM-2 plasma sensor at which these plasma characteristics are stored. Nonetheless, due to their complexity, CHARM-2 plasma sensors are expensive to fabricate. Moreover, neither CHARM® plasma sensors nor CHARM-2 plasma sensors may be used to evaluate a plasma in real time.
U.S. Pat. No. 6,144,037, issued to Ryan et al. on Nov. 7, 2000 (hereinafter “Ryan”), discloses a capacitor charging sensor that, purportedly, more closely imitates the features of a semiconductor substrate during exposure thereof to plasma processes than do CHARM® and CHARM-2 plasma sensors. Nonetheless, the capacitor charging sensor of Ryan is not useful for monitoring the effects of a plasma on a semiconductor substrate in real time. Further, as with CHARM® plasma sensors, the usefulness of CHARM-2 plasma sensors in monitoring plasmas is limited to the two dimensions along the surfaces of such sensors.
While apparatus and methods for monitoring plasma electric potentials in real time is known, the actual electric currents and potentials and, thus, the quality of plasma processing are directly impacted by the dimension of each feature being processed.
Nonetheless, the inventor is not aware of any real time plasma monitoring devices or methods that facilitate measurement of electric potentials at structures that emulate processing of semiconductor device features at a sub-micron scale. Further, the inventor is not aware of plasma probes that are capable of monitoring a plasma in three dimensions.